How to get 14 YZ250F to shift smooth?

Try a higher end synthetic oil, and if that doesn't fix it, go the ISF route.

You may have over tightened the bolt that holds the shift stopper in place. I did and it was a pain in the a** to shift so I opened it up again and torqued it to the exact amount, 8ft lbs I think, and it shift just fine now. Check that first.

Going back 12 years, I bought Motorex, for my KTM. I believe that is their factory fill oil, but not sure if they used the synthetic, at that time. I can't really give you a specific recommendation, I'm somewhat out of the loop now, but maybe try the Rotella synthetic for diesels. Heard it has a high nickel content, good for the trans, and cushioning the clutch basket. Maybe just buy 1 small bottle to try it, and if it works, buy the big jug. Hell, if it was me, I'd buy the big pail, if it's the most cost effective. That's just me, though.

Regular white bottle Rotella has been working great in my 14 Yz250f with the updated shift stopper. I change it every 3-4 hours.